The present invention relates to an optical signal transmitter-receiver module of the class wherein there are arranged on an identical substrate, a light emitting element and a light receiving element.
Regarding the optical signal transmitter-receiver system, especially for connecting its central office with subscribers, it has been pointed out that to more popularize the system, the optical signal transmitter-receiver module to be installed on the subscriber side should be preferably more compact and be available at a lower cost. A technical report entitled "Present State and Future of Hybrid Optical Integration Technology Using PLC Platform," published by the Institute of Electronics, Information and Communication Engineers of Japan, EMD96-24, discusses an optical signal transmitter-receiver module that could respond to such pointing out. This optical signal transmitter-receiver module includes at least one light emitting element, at least one light receiving element, and an optical waveguide, all of which are mounted on an identical substrate.
At present, such optical signal transmitter-receiver module is still discussed as to whether it is applicable or not, to the synchronous transfer mode passive double star (STM-PDS) system and to the asynchronous transfer mode passive double star (ATM-PDS) system. The STM-PDS system is such a system that operates the optical signal transmitter-receiver module such that it transmits and receives the optical signal in the time sharing mode. On one hand, the ATM-PDS system is such a system that operates the optical signal transmitter-receiver module such that it transmits and receives the optical signal asynchronously.
A prior art optical signal transmitter-receiver module 200 will now be explained in the following with reference to FIGS. 37 through 39 of the accompanying drawings. In these figures, FIG. 37 is a perspective view of the prior art optical signal transmitter-receiver module 200. FIG. 38 is a plan view of the optical signal transmitter-receiver module 200. FIG. 39 is a circuit diagram of an optical signal transmission-reception circuit 205 of the optical signal transmitter-receiver module 200.
As shown in FIG. 37, the optical signal transmitter-receiver module 200 transmits and receives the optical signal having a wavelength of lambda 1 in the time sharing mode. However, it makes the optical signal having a wavelength of lambda 2 transfer to the outside, this optical signal being inputted asynchronously with the transmission-reception of the optical signal having a wavelength of lambda 1.
In the following description and the accompanying drawings, the transmitting optical signal having a wavelength of lambda 1 will be referred to and indicated as "optical signal Pt" while the receiving optical signal with the wavelength of lambda 1 will be referred to and indicated as "optical signal Pr." Furthermore, in this specification and the accompanying drawing, the optical signal having a wavelength of lambda 2 that is indifferent to transmission-reception will be referred to and indicated as "optical signal Pu."
As shown in FIG. 38, the optical signal transmitter-receiver module 200 includes a planar lightwave circuit (PLC) 201, and a signal transmission-reception circuit 205. PLC 201 is composed of a Si substrate 2, a quartz layer 3 formed on the Si substrate 2, an optical waveguide passage 204 formed on the quartz layer 3, a branch passage 204d that is formed in the optical waveguide passage 204 for use in separating the wavelength, a dielectric interference filter 5 arranged on the branch passage 204d, a laser diode (referred to as LD hereinafter) 206 mounted on the surface of the Si substrate 2 and for transmitting the optical signal, a photodiode for monitoring the transmitting optical signal (referred to as m-PD hereinafter) 202 that is mounted on the surface of the Si substrate 2, and a photodiode for receiving the optical signal (referred to as r-PD hereinafter) 203 that is mounted on the surface of the Si substrate 2.
The optical waveguide passage 204 includes an input/output port 204a, an output port 204b, a Y-shaped branch passage 204c, a branch passage 204d, a signal transmitting port 204e, and a signal receiving port 204f. An optical fiber (not shown) for transmitting the optical signal is arranged in the vicinity of the input/output port 204a. Optical signals Pr and Pu transmitted through the above-mentioned optical fiber are inputted to the input/output port 204a. The optical signal Pt generated from the LD 206 is emitted from the input/output port 204 directing to the above-mentioned optical fiber. An optical module (not shown) is arranged in the vicinity of the output port 204b in order to receive the optical signal Pu.
The dielectric interference filter 5 is provided at the branch passage 204d of the optical waveguide passage 204 such that it is buried in a slot formed in the quartz layer 3. This dielectric interference filter 5 allows the optical signal having a wavelength of lambda 1 to pass therethrough, but it reflects the optical signal having a wavelength of lambda 2 inputted through the input/output port 204a, directing to the output port 204b.
The LD 206 is arranged such that its front face is located in the vicinity of the signal transmitting port 204e and emits the optical signal Pt. This optical signal Pt emitted from the LD 206 is inputted to the signal transmitting port 204e. The m-PD 202 is arranged such that its front face is located in the vicinity of the backside face of the LD 206 and receives the optical signal Pm emitted therefrom. The r-PD 203 is arranged such that its front face is located in the vicinity of the signal receiving port 204f and receives the optical signal Pr outputted therefrom.
The signal transmission-reception circuit 205 is composed of a signal transmitting circuit 11 and a signal receiving circuit 12. The signal transmitting circuit 11 drives the LD 206 responding to the transmitting electric signal, keeping the light emission power of the LD 206 constant. As shown in FIG. 39, this signal transmission-reception circuit 205 includes a flip-flop (referred to as F/F hereinafter) circuit 13, an LD driving circuit 14, and an automatic power control (referred to as APC hereinafter) circuit 15. The F/F circuit 13 latches the transmitting electric signal directed to the LD driving circuit 14. The LD driving circuit 14 supplies the LD 206 with a driving current corresponding to the transmitting electric signal given by the F/F 13, and drives the LD 206. The APC circuit 15 controls the driving current supplied from the LD driving circuit 14 to the LD 206 depending on the quantity of the optical signal Pt (emitted from the backside of LD 206) received by the m-PD 202, thereby keeping the light emission power of the LD 206 constant.
The m-PD 202 and the APC circuit 15 are provided just for the purpose of keeping the light emission power of the LD 206 constant. Therefore, should it be enough to consider only the function of transmitting and receiving the optical signal having a wavelength of lambda 1, there might be no need for the m-PD and the APC circuit to be prepared. However, the light emission power of the LD 206 is considerably varied by the ambient temperature even though the driving current is kept constant, so that in order to keep the light emission power of the LD 206, it is needed to adjust the driving current in response to variation of the ambient temperature. For this, it is required that the signal transmission-reception circuit 205 monitors the light emission power of the LD 206 by means of the m-PD 202 and controls the driving current by means of the APC circuit 15 such that difference between the monitored current coming from the m-PD 202 and a reference current value becomes as small as possible, thereby keeping the light emission power of the LD 206 constant.
The signal receiving circuit 12 generates the receiving electric signal depending on the quantity of the optical signal Pr received by the r-PD 203. This signal receiving circuit 12 is composed of a preamplifier 16, an automatic threshold value control (referred to as ATC hereinafter) circuit 17, and a limiting amplifier (referred to as LIM hereinafter) circuit 18. The preamplifier 16 converts the receiving current inputted by the r-PD 203 into a receiving voltage, which is then outputted to the ATC circuit 17. This ATC circuit 17 automatically sets a threshold voltage against the receiving voltage as inputted thereto, adjusts (clips) this receiving voltage referring to the threshold value, and then outputs it to the LIM circuit 18. The LIM circuit 18 converts the receiving voltage given by the ATC circuit 17 into the receiving electric signal in terms of the logic level.
In the next, there will be explained the operation of the above prior art optical signal transmitter-receiver module 200. The optical signal having a wavelength of lambda 1 is transmitted and received in the time sharing mode by the optical signal transmitter-receiver module 200. The optical signal having a wavelength of lambda 2 is always inputted to the input/output port 204a and guided to the output port 204b. FIG. 40 is a timing chart for explaining the transmission-reception operation of the optical signal having a wavelength of lambda 1 by means of the prior art optical signal transmitter-receiver module 200. In FIG. 40, the optical signal PI/0 having a wavelength of lambda 1 that comes in and out from the input/output port 204a, is the optical signal Pt during the period of signal transmission mode while being the optical signal Pr during the period of signal reception mode.
During the period of signal transmission mode for transmitting the optical signal having a wavelength of lambda 1, the transmitting electric signal is latched by the F/F circuit 13 of the signal transmitting circuit 11 on the bit by bit basis and is then inputted to the LD driving circuit 14. The LD driving circuit 14 supplies the LD 206 with the driving current ILD corresponding to the transmitting electric signal that is inputted from the F/F circuit 13, thereby driving the LD 206 to turn it on and off. In this way, the LD 206 generates the optical signal Pt corresponding to the transmitting electric signal. The optical signal Pt emitted from the front face of the LD 206 is inputted to the signal transmitting port 204e.
About 3[dB] (50%) of the optical signal Pt incident on the signal transmitting port 204e leaks outward at the Y-shaped branch passage 204c, and only the remaining 3[dB] thereof is guided up to the dielectric interference filter 5. This optical signal Pt receives the optical loss of about 2[dB] when it passes through the dielectric interference filter 5 and is then emitted from the input/output port 204a. Thus, the optical signal Pt has to be subject to the optical loss of about 5[dB] eventually until it is guided up to the input/output port 204a after incidence on the signal transmitting port 204e.
During the period of signal transmission mode, the optical signal Pt emitted from the backside face of the LD 206 is inputted to the m-PD 202 and converted into the electric current (i.e. a monitoring current ImPD) thereby. This monitoring current ImPD is such a current that has a magnitude corresponding to the light emission power of the LD 206 and is inputted to the APC circuit 15 of the transmitting circuit 11. The APC circuit 15 controls the driving current ILD supplied from the LD driving circuit 14 to the LD 206 such that the monitoring current ImPD is kept constant, thereby keeping the light emission power of the LD 206 constant.
In the next, during the period of signal reception mode for receiving the optical signal having a wavelength of lambda 1, optical signals Pr and Pu are inputted to the input/output port 204a through the optical fiber. The optical signal Pu incident on the input/output port 204a is reflected by the dielectric interference filter 5 and is emitted from the output port 204b.
On one hand, the optical signal Pr incident on the input/output port 204a is allowed to pass through the dielectric interference filter 5. At this time, however, the optical signal Pr receives the optical loss of about 2[dB]. The optical signal Pr having passed through the dielectric interference filter 5 is divided into two parts of about 3[dB] each at the Y-shaped branch passage 204c of the optical waveguide passage 204. One of the divided optical signals Pr is guided to the signal receiving port 204f, then inputted to the r-PD 203, and finally converted into a current (receiving current IrPD) thereby. The other of the divided optical signals Pr is guided to the signal receiving port 204e and is treated there as a redundant current doing nothing to signal receiving operation. Consequently, the optical signal Pr receives the optical loss of about 5[dB] until it propagates to the r-PD 203 through the input/output port 204a and the optical waveguide 204.
The above-mentioned receiving current ImPD is inputted to the preamplifier 16 of the signal receiving circuit, and converted into a corresponding voltage thereby, and further converted into the receiving electric signal in terms of logic level by means of the ATC circuit 17 and the LIM circuit 18.